It is well recognized that the problems of reproducing sound with high fidelity and clarity involve a multiplicity of subtle factors. The instrument or human organ whose sound has been recorded is not a pure or monofrequency sound source, the waveforms involved are complex, intermittent, and at times asymmetrical, and the transducer which is to reproduce the sound is necessarily a different type of sound source than the original. The transducer is moreover a complex electromechanical mechanism which exhibits its own resonance and mode characteristics under excitation; and the media and structure into which the forward and backward waves are transmitted react with the transducer to affect both transducer operation and the quality of the sound. It is, of course, feasible to use large horns or other loudspeakers for improved efficiency and for better impedance matching to the sound-receiving volume. However, the cost and size of such speaker systems limits their use to a relatively small proportion of the total number of installations. The great majority of high fidelity audio installations comprise a set of transducers, each excited in a different frequency range, such as the common woofer, mid-range and tweeter combination.
Many studies have been undertaken of the response and distortion characteristics of loudspeaker systems and the specifications of the speakers are usually stated in terms of linearity of response over the frequency band, as well as various measurable distortions. These data are commonly derived from analyses made using pure excitation frequencies, singly or in limited combinations, with the assumption being made that the resultant graphs and figures of merit establish performance quality for all modes of operation. It is well recognized, however, that human responses are based upon much more arcane and complicated evaluative factors. The loudspeaker must be regarded as an imperfect mechanism in comparison to the ear, which is a profoundly competent instrument that is responsive to almost immeasurable differences in sound reproduction. Thus, just as the seeing organs are acutely sensitive to visible wavelength differences, the ear is sensitive to minute phase, pitch, resonance and shading effects in high fidelity reproduction of complex sounds. A major advance in this art does not therefore entail an improvement in multiples or orders of magnitude, but only a fractional or low percentage improvement, if it is of a quality that is detectable by the ear.
As far as is known, workers in the art have not significantly considered the differences in character and dynamics of sound emanating from different types of sources. The typical small loudspeaker is, in the parlance of the art, described as a simple source or point source, as set forth in Chapter 4 in the book "Acoustics" by L. L. Beranek, published in 1954 by the McGraw-Hill Book Company, a basic work in the field. Other sources, such as the sounding board of a piano, are much more complicated generators of sound, and are typically much longer with relation to the wavelength of many of the sounds that are generated. While a loudspeaker cone is typically less in diameter than one-fourth of the wavelength of the sound which it must generate, the sounding board of a piano can be much longer than such wavelengths. Thus, a piano sounding board here may be termed an extended source, to distinguish it from the simple source. Workers in the art have heretofore analyzed extended sources of sound on a theoretical and steady state basis in terms of an array of point sources which act by diffraction and interference effects to provide a principal radiation lobe and side lobes which can be characterized in general terms. However, this type of analysis does not suffice for a highly interactive structure such as a piano sounding board when excited by multi-frequency waves which furthermore can be intermittent in character. A wholly different type of extended source is the human voice, which has both variable excitation organs and a variable sound chamber. Such extended sources generate sounds which contain unidirectional components, varying phase components, and transient effects, which may be visualized as sharp leading and trailing edge waveforms. Thus, it is not correct to try to envision the total interactive response of a transducer in terms of measurable responses to pure steady state single frequencies or combinations of frequencies from a standardized oscillator or other pure source. In this connection, one can recognize that the accurate reproduction of normal human speech is extremely difficult, and that even an idiosyncratic high fidelity enthusiast accepts as normal a substantial deterioration in reproduction quality from this type of source. It has ascertained, as discussed in detail below, that the mentioned factors in complex acoustic programming material cause spurious simple source emanations when attempted to be reproduced in conventional speaker systems. The spurious simple source emanations are inherently accepted by listeners as inevitable, until exposed to sound reproduction from which the emanations are absent. The present invention represents both a discovery of the causes and character of spurious simple source emanations and a teaching of various practical resolutions of the problem.